内蒙古西乌旗梅劳特乌拉蛇绿岩的识别

新识别出的内蒙古西乌旗梅劳特乌拉蛇绿岩位于中朝板块与西伯利亚板块之间的兴蒙造山带北部,从空间展布看,为二连浩特-贺根山蛇绿岩带的东延部分,通过对梅劳特乌拉蛇绿岩进行详细的野外地质调查和岩石学、地球化学、年代学的研究,发现梅劳特乌拉蛇绿岩带呈ENE-NE向展布,宽约6~11km,延伸约24km,蛇绿岩各单元出露较齐全,岩性主要为蛇纹石化方辉辉橄岩、层状-块状辉长岩、辉绿岩脉(墙)、枕状玄武岩、辉斑玄武岩、及硅质岩。蛇纹石化方辉辉橄岩具有低SiO 2、高MgO,稀土配分模式显示宽缓的U型,具SSZ(supra-subduction zone)型蛇绿岩的地幔橄榄岩特征。枕状玄武岩和辉绿岩脉(墙)具低K2O(平均0.82%)和TiO2(0.8%~1.2%)的特征,球粒陨石标准化显示LREE弱亏损的平坦型;微量元素MORB标准化蛛网图显示:枕状玄武岩和辉绿岩脉(墙)的大离子亲石元素Sr、K、Rb和Ba含量明显富集,而高场强元素Nb、Th、Ta、Zr、Ti明显亏损,为典型的SSZ构造背景成因的熔岩特征。获得块状辉长岩的LA-ICP-MS锆石U-Pb年龄为308.5±2.2Ma,据此确定梅劳特乌拉蛇绿岩形成于晚石炭世,为约束兴蒙造山带的构造演化提供了新佐证。

Recognition of Meilaotewula ophiolite in Xi U jimqin Banner, Inner Mongolia

Firstly recognized Meilaotewula ophiolite occurs in Xing'anling-Mongolian Orogenic Belt (XMOB) between the Sino-Korean Paleoplate and the Siberian Paleoplate in the central sector of Inner Mongolia. Based on the regional geology, the Meilaotewula ophiolite can be the eastward extension of the Erlianhaote-Hegenshan ophiolite zone. According to field geological investigation and the preliminary results of petrology and geochemistry, we found Meilaotewula ophiolite is about 6~11km wide, and extends 24km in a ENE-NE direction, and it outcrops completely, including augite peridotite, beded-massive gabbro, diabase, pillow basalt, augitophyre, and chert. The augite peridotites are characterized by low SiO₂, high MgO, U-shaped chondrite-normalized REE patterns, represent a typical mantle section of the supra-subduction zone (SSZ) setting. Pillow basalts and diabase are characterized by low K₂O (average 0.82%) and TiO₂ (0.8%~1.2%), and have flat to slightly LREE-depleted chondrite-normalized REE patterns. In MORB-normalized spider diagrams, pillow basalts and diabase are enriched in Sr, K, Rb and Ba, and depleted in Nb, Th, Ta, Zr and Ti, displaying the affinity of the supra-subduction zone (SSZ) setting. LA-ICP-MS U-Pb dates on zircon of massive gabbro yield ages of 308.5±2.2Ma, which is the crystallizatioan age of gabbro, indicating a Late Carboniferous age of Meilaotewula ophiolite. All the research results above provide important evidence in exploring the tectonic evolution of Xing'anling-Mongolian Orogenic Belt (XMOB).